Defect classification utilizing data from a non-vibrating contact potential difference sensor

ABSTRACT

A method and system for identifying and classifying non-uniformities on the surface of a semiconductor or in a semiconductor. The method and system involves scanning the wafer surface with a non-vibrating contact potential difference sensor to detect the locations of non-uniformities, extracting features characteristic of the non-uniformities, and applying a set of rules to these features to classify the type of each non-uniformity.

FIELD OF THE INVENTION

The present invention is directed to methods and systems for theinspection of surfaces and materials, including semiconductor surfacesand semiconductor materials. More particularly, the present invention isdirected to methods for the classification of surface or subsurfacenon-uniformities and/or charges detected using at least one of avibrating and a non-vibrating contact potential difference sensor.

BACKGROUND OF THE INVENTION

The function, reliability and performance of semiconductor devicesdepend on the use of semiconductor materials and surfaces which areclean and uniform. Billions of dollars and countless man-hours have beenspent developing, characterizing, and optimizing systems and processesfor fabricating and processing semiconductor materials. A primary goalof this activity has been the fabrication of materials and surfaces thatare extremely clean and that have properties that are uniform, or varyuniformly, across the entire wafer. In order to characterize andoptimize these processes it is necessary to be able to inspect andmeasure surface or bulk cleanliness and uniformity. For real-timeprocess control, it is necessary to be able to make many measurementsacross a surface at high speed, and to do so in a manner that does notdamage or contaminate the semiconductor surface. It is also highlydesirable to be able to discriminate and classify different types ofnon-uniformities or contaminants. Classification is extremely important.Information on the nature of a non-uniformity can be used to determineif the non-uniformity might impact device performance or manufacturingyield. Classification information can also be used to identify thesource of the non-uniformity.

One method of inspecting and measuring surfaces utilizes a non-vibratingcontact potential difference sensor. The non-vibrating contact potentialdifference sensor consists of a conductive probe that is positionedclose to a surface, and is electrically connected to the surface. Theprobe and the surface form a capacitor. A potential difference is formedbetween the probe tip and the surface due to the difference in workfunctions or surface potentials of the two materials. The probe tip istranslated parallel to the surface, or the surface is translated beneaththe probe. Changes in the work function or surface potential atdifferent points on the surface result in changes in potential betweenthe surface and the probe tip. Also, changes in the distance between theprobe tip and the wafer surface results in changes in the capacitance.Changes in either the potential or capacitance between the probe tip andthe wafer surface causes a current to flow into the probe tip. Thiscurrent is amplified, converted to a voltage, and sampled to form acontinuous stream of data which represents changes across the surface.The non-vibrating contact potential difference sensor can provide acontinuous stream of data at rates greater than 100,000 samples persecond. High data acquisition rates permit high-resolution images ofwhole semiconductor wafers to be acquired in only a few minutes.

The non-vibrating contact potential difference sensor produces a signalthat is a combination of two characteristics of the measuredsurface-changes in surface potential and changes in surface height. Thecharge on the probe tip is determined as follows:

Q=CV (1)

Where Q is the charge on the probe tip, C is the capacitance between theprobe tip and the measured surface, and V is the potential differencebetween the probe tip and the surface.

The current, i, into the probe tip is the derivative of the charge onthe probe tip and is given by the following formula:

$\begin{matrix}{i = {\frac{Q}{t} = {{C\frac{V}{t}} + {V\frac{C}{t}}}}} & (2)\end{matrix}$

The current, i, is the sum of two terms: the dV/dt term and the dC/dtterm. The dV/dt term represents changes in the voltage between the probetip and the wafer surface, and the dC/dt term represents changes in thecapacitance between the probe tip and the wafer surface. The potentialof the probe tip is fixed during the scanning operation, so changes inthe dV/dt term arise from changes in the potential of the measuredsurface. Changes in capacitance result from changes in the distancebetween the probe tip and the wafer surface, which most often resultfrom changes in the height of the wafer surface. This formulaillustrates how the current into the sensor is a combination of changesin the potential and height of the measured surface.

Changes in surface potential can result from a variety of changes insurface and subsurface conditions. These include; but are not limitedto; contamination by metals, contamination by non-metals or organics,changes in surface chemistry, changes in the number or type of moleculesfrom the environment that adsorb on the surface, changes in the chemicaltermination of the surface, charging on the surface, charging in adielectric deposited on the surface, changes in the atomic roughness ofthe surface, changes in surface film stress, changes in subsurfacedoping density or implant dose, changes in subsurface doping or implantdepth, changes in potential at subsurface interfaces, changes insubsurface electrical conductivity, changes in crystalline structure ordamage, changes in surface illumination, or some combination of thesefactors.

The non-vibrating contact potential difference sensor system can detecta wide range of surface and subsurface non-uniformities. However, itwould be desirable to enhance the capabilities of the non-vibratingcontact potential difference sensor inspection system to enable thediscrimination and classification of different types ofnon-uniformities. For example, it would be desirable to discriminatebetween surface potential non-uniformities resulting from metalliccontamination and surface potential non-uniformities resulting fromorganic contamination. As a second example, it would be desirable todiscriminate between surface chemical contamination and variations inthe height of the wafer surface.

SUMMARY OF THE INVENTION

The system and methods described in this invention provide an enhancedapplication of at least one of a vibrating and a non-vibrating contactpotential difference inspection system that allows the discriminationand classification of different types of surface and subsurfacenon-uniformities. Hereinafter, material susceptible to inspection by thesystem herein described will be denoted generally as a “wafer”. Theinvention includes at least one of a vibrating and a non-vibratingcontact potential difference sensor scanning system that generates datarepresentative of surface potential and height variations across thewafer surface. This apparatus consists of at least one of a vibratingand a non-vibrating contact potential difference sensor, a system formechanically fixturing the wafer, a system for positioning the sensor afixed distance above the wafer surface and generating relative motionbetween the probe tip and wafer surface such that the sensor probe tipmoves parallel to the wafer surface, and a system for acquiring andprocessing the output signal from the sensor to identify and classifywafer non-uniformities.

In one embodiment, the system includes the ability to apply a biasvoltage to either the sensor probe tip or the wafer surface to modifythe electrical potential between the probe tip and wafer. In this casethe dC/dt term in equation 2 includes a bias voltage as shown in thefollowing formula:

$\begin{matrix}{i = {{C\frac{V}{t}} + {\left( {V_{CPD} + V_{bias}} \right)\frac{C}{t}}}} & (3)\end{matrix}$

In equation 3, V_(CPD) is the voltage between the probe tip and wafersurface that results solely from electrically connecting the probe tipand the wafer surface. This voltage is called the Contact PotentialDifference, or CPD. V_(bias) is an additional voltage that is applied tothe probe tip or wafer by the inspection system to facilitate detectionand classification of wafer non-uniformities. If V_(bias) is constantduring the scanning operation, then it does not affect the dV/dt termbecause dV_(bias)/dt=0.

In an alternative embodiment the system includes a mechanism forpositioning the sensor above a point on the wafer and vibrating thesensor perpendicular to the wafer surface while the bias voltage isadjusted. Vibrating the probe tip perpendicular to the wafer surfacecauses changes in the capacitance between the probe tip and the wafersurface, which results in a signal due to the VdC/dt term in equations 2and 3. This signal is proportional to the contact potential difference(V) between the probe tip and the wafer surface. The variable bias isadjusted to determine the bias voltage that matches the potential of theprobe tip to the potential of the wafer surface. When the bias voltageis equal and opposite to the contact potential difference between theprobe and the wafer then the signal from the sensor goes to zero. Byadjusting the bias voltage, the bias that results in zero signal isdetermined, and the contact potential difference is calculated as thenegative of this bias voltage. This method of operation is known as avibrating Kelvin probe or Kelvin-Zisman probe.

In another alternative embodiment the system includes the ability tocontrol the illumination of the wafer surface near the probe tip. Thiscapability allows the wavelength, light intensity or angle ofillumination to be configured prior to, as well as during, scanning thesurface using the non-vibrating contact potential difference sensor. Inyet another alternative embodiment the system includes the ability todeposit charge on the wafer surface prior to the surface being scannedby the non-vibrating contact potential difference sensor. Anon-uniformity can also be evaluated by measuring a charge which isdeposited or otherwise present on the wafer, or on or in a dielectriclayer on the wafer. The non-uniformity can include a charge itself on orin the wafer, or on or in the dielectric layer itself. In addition achange of signal arising from the deposited charge can enableidentification of the type and amount of the non-uniformity.

The invention includes a system and methods for processing the resultingnon-vibrating contact potential difference data to discriminate betweena wide range of different types of surface non-uniformities. In analternative embodiment, the system includes the ability to obtainmultiple scans of the same wafer, or multiple scans of the same part ofthe same wafer. Each of the multiple scans can be acquired withdifferent bias voltages, illumination wavelengths, illuminationintensities, illumination angles; and/or with different amounts ofcharge applied to, or otherwise present on the surface or embedded intoselected layers of a wafer and deposited layers, such as dielectrics.Data from these multiple scans can be processed separately or combinedto discriminate between different types and relative amounts ofnon-uniformities, such as chemical, mechanical and charges.

The non-vibrating contact potential difference data is processed toidentify regions of wafer non-uniformity. In one embodiment, this isaccomplished by thresholding the differential non-vibrating contactpotential difference image. Thresholding involves identifying those datapoints with specific values, where the specific values are defined asthose signal values that are greater than a particular value, less thana particular value, or within some range of values. The particularvalues used to define areas of wafer non-uniformity are referred to asthresholds. In an alternative embodiment, non-uniformities areidentified by integrating the differential data to form an image thatrepresents relative surface potential values. The resulting integratedimage can be thresholded to identify regions with specific relativesurface potential values or ranges of values. Once regions ofnon-uniformity are identified, additional processing is performed toclassify each region.

In one embodiment, the integrated image data for regions ofnon-uniformity are compared to the integrated image data for other partsof the wafer to determine if the non-uniformity results in a positive ornegative change in surface potential or capacitance. If a non-uniformityrepresents a change in surface potential, then the direction (positiveor negative) of the signal indicates whether the surface potential isincreasing or decreasing. If a non-uniformity represents a change insurface height, then the direction of the signal indicates whether thewafer height is increasing or decreasing. This information is then usedto classify non-uniformities based on the direction of surface potentialor surface height change. For example, surface contamination may resultin an increase or decrease in wafer surface potential. Many, but notall, types of metal contamination on silicon surfaces result in anincrease in surface potential, while many types of organic contaminationon silicon surfaces results in a decrease in surface potential. Ininspection applications where likely metal contaminants increase surfacepotential and likely organic contaminants decrease surface potential,the direction of surface potential change can be used to distinguishbetween metallic surface contamination and organic surfacecontamination. Independent data can be used to assist in determining thelikelihood of contaminants or non-uniformities being of a particularclass, such as metal, non-metal, semiconductor dopants, insulators andionic impurities. Such independent data can arise from use of otherconventional devices, such as SEM, X-ray fluorescence, ion probe, andstraightforward evaluation of the likely contaminants from a particularmanufacturing process.

The signal component that results from changes in the distance betweenthe probe tip and the wafer surface varies linearly with the voltagebetween the probe tip and the wafer surface, while the signal componentresulting from changes in surface potential is unaffected by a constantbias voltage. As a result, the same wafer can be scanned twice with twodifferent bias voltages, and the resulting images subtracted from eachother, to form an image that has minimal signal from surface potentialchanges and an increased signal from height changes. This is illustratedby the following equations:

$i_{1} = {{C\frac{\left( V_{CPD} \right)}{t}} + {C\frac{\left( V_{{bais}\; 1} \right)}{t}} + {\left( {V_{CPD} + V_{{bias}\; 1}} \right)\frac{C}{t}}}$$i_{2} = {{C\frac{\left( V_{CPD} \right)}{t}} + {C\frac{\left( V_{{bais}\; 2} \right)}{t}} + {\left( {V_{CPD} + V_{{bias}\; 2}} \right)\frac{C}{t}}}$$i_{1 - 2} = {\left( {V_{{bias}\; 1} - V_{{bias}\; 2}} \right)\frac{C}{t}}$$\frac{\left( V_{{bias}\; 1} \right)}{t} = 0$$\frac{\left( V_{{bias}\; 2} \right)}{t} = 0$

In this case, i₁ is the current into the probe tip when a bias voltageof V_(bias1) is applied, and i₂ is the current into the probe tip when abias voltage of V_(bias2) is applied. Because the derivative of the biasvoltages is 0, subtracting the two currents results in a new signal thatis unaffected by changes in contact potential difference and is formedsolely from changes in capacitance between the probe tip and wafersurface. The resulting signal or image can be used to detect andclassify non-uniformities as variations in wafer height as opposed tochanges in wafer surface potential. Likewise, non-uniformities thatappear in an image acquired with no bias voltage, but do not appear inan image that is formed by subtracting two images acquired withdifferent bias voltages, can be classified as a surface potential changeas opposed to a surface height change.

The average contact potential difference between the probe tip and thewafer surface can be determined by making vibrating Kelvin probemeasurements at one or more points on the wafer surface. A bias voltagewhich is equal in magnitude, but opposite in polarity, can then beapplied to the probe tip to minimize the average difference in potentialbetween the probe tip and wafer surface. The wafer can then be scannedusing this bias voltage, and the signal component due to changes in thedistance between the probe tip and the wafer surface will be minimized.Subsequent wafers with similar surfaces can also be scanned using thesame bias voltage without the need to make additional vibrating Kelvinprobe measurements. The use of a bias voltage which minimizes theaverage potential between the probe tip and the wafer surface permitsthe formation of a signal and image which can be used to identifynon-uniformities in surface potential while minimizing the signal due tochanges in the height.

One advantage of the non-vibrating contact potential difference sensoris that it can acquire data relatively quickly, so that whole-waferimages can be acquired in only a few minutes. The availability ofhigh-resolution images permits the use of image processing algorithms toclassify non-uniformities based on features associated with their shapeor signal levels. For example, the differential or integrated image canbe thresholded, and the resulting defect map can be segmented toidentify connected regions of non-uniformity. Each region can then beassociated with a list of features; such as area, perimeter, height,width, signal range, average signal value, minimum and maximum signalvalues, and many others. These features can be calculated from thedifferential or integrated images of the non-uniformities, or fromimages acquired with different bias voltages, illuminationconfigurations, surface charge conditions, or combinations of theseimages. This list of features can then be used to classify the defectbased on a mathematical algorithm or set of rules. For example, in thesimplest case non-uniformities can be classified based on size orstandard deviation of the non-vibrating contact potential differencesensor signal.

The ability to control the intensity, wavelength, and angle ofillumination; and the amount of charge on the wafer surface or embeddedin dielectric or other layers present on the wafer provides additionalmethods of classifying defects or location and amounts of charge. Thesurface potential change resulting from some types of defects issensitive to surface illumination or charging. Illumination can be usedto vary the surface potential by reducing the effect of electric fieldsin the semiconductor, while charge deposited or otherwise present on thesurface, or in or on a dielectric layer, can be used to induce electricfields and vary the distribution of charge within the semiconductor. Theability to control the illumination and surface charge allows two ormore scans to be acquired with different illumination and/or chargeconditions. These scans can then be combined mathematically, for exampleby subtracting one scan image from another, to determine the change insignal due to changes in illumination or charge. Surfacenon-uniformities can then be classified based on the magnitude andpolarity of the change in signal that results from changing illuminationor changing surface charge conditions.

The vibrating Kelvin probe measurement capability provides yet anothermethod of classifying defects. Once a defect has been detected andlocated within a non-vibrating contact potential difference image, oneor more vibrating Kelvin probe measurements can be made to determine thecontact potential difference at the location of the defect. Theresulting contact potential difference data can be correlated tospecific types or concentrations of defects, and this information can beused to identify or quantify the detected defect. Also, vibrating Kelvinprobe measurements can be made with different surface charge orillumination conditions to determine the absolute change in surfacepotential with changing illumination variables or charging of the waferor other attached layer. This information can also be used to classifythe type of defect.

Another method of defect classification involves integrating the signalobtained from the non-vibrating contact potential difference sensor, andthen making two or more vibrating Kelvin probe measurements at differentpoints on the wafer. The integrated signal represents relative surfacepotential values, but does not represent absolute contact potentialdifference values. The vibrating Kelvin probe measurements can be usedto scale the integrated signal values so that they represent absolutecontact potential difference values. This is accomplished by calculatinga linear or curve fit between the vibrating Kelvin probe measurementsand the signal values at the same locations within the integrated image.This transformation is then applied to all integrated signal values. Theresulting integrated image values can then be used to classify surfacenon-uniformities based on their absolute contact potential differencevalues.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a wafer inspection system with a non-vibratingcontact potential difference sensor, a source of deposited charge, asource of controlled illumination and a system for vibrating the probeperpendicular to the wafer surface to make vibrating Kelvin probemeasurements, the system including a component for processing data fromthe sensor, automatically detecting surface or sub-surfacenon-uniformities, and classifying the non-uniformities;

FIG. 2A shows a diagram on the left with a radial scanning system and inFIG. 2B an image on the right created by the scanning operation;

FIG. 3A shows an image created by scanning a wafer with a non-vibratingcontact potential difference sensor, and FIG. 3B is an image afterthresholding where the dark regions in the thresholded image representnon-uniformities;

FIGS. 4A-4C show three different images where FIG. 4A was formed byintegrating the signal in the image in FIG. 3A; FIG. 4B shows the imageafter applying a threshold to identify areas of increased surfacepotential; and FIG. 4C shows the image after thresholding to identifyregions of decreased surface potential;

FIGS. 5A-5C show three images, and the image of FIG. 5A was created byscanning a wafer with a non-vibrating contact potential differencesensor with no surface illumination; FIG. 5B was created by scanning thesame wafer with the surface illuminated by visible light; and FIG. 5C isthe difference between the images of FIG. 5A and FIG. 5B;

FIGS. 6A-6C show three images, and the image of FIG. 6A was acquired byscanning a wafer with a non-vibrating contact potential differencesensor with a 5 Volt bias applied to the probe tip; the image of FIG. 6Bis an image of the same wafer with a −5 Volt bias applied to the probetip,and the image of FIG. 6C is the difference of the images of FIGS. 6Aand 6B;

FIG. 7 shows an image of a wafer acquired by scanning with anon-vibrating contact potential difference sensor;

FIG. 8 shows Absolute CPD Measurements (ACMs) acquired with a vibratingKelvin probe; and

FIGS. 9A-9D show four images of the same wafer acquired by scanning thewafer with a non-vibrating contact potential difference sensor; theimage of FIG. 9A is acquired with no bias voltage applied between theprobe tip and the wafer surface; the image of FIG. 9B was acquired witha 5 Volt bias applied to the probe tip; and the image of FIG. 9C shows aheight change feature of the wafer; and the image of FIG. 9D was formedby using the vibrating Kelvin probe mode to measure the contactpotential difference between the probe tip and the wafer surface atmultiple points, and then applying the negative of the average contactpotential difference to the probe tip.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides an enhanced inspection system that uses anon-vibrating contact potential difference sensor system to detectsurface and sub-surface non-uniformities, including deposited (orotherwise present) charge, and a system for processing data from thenon-vibrating contact potential difference sensor to classify differenttypes and relative amounts of non-uniformities. This invention is notlimited to the measurement of semiconductors with bare, clean surfaces.The chemical state of the surface may vary, or surface contamination maybe present. Also, the wafer surface may be covered with a coating orfilm. For example, a silicon wafer surface is often coated with asilicon oxide or other dielectric or metal film. This invention can beused to inspect a wafer covered with a film to detect defects in theunderlying semiconductor or at the semiconductor-film interface. Inaddition, this invention can be used to detect or classify defects in,or on, a film.

Referring to FIG. 1, the apparatus consists of a non-vibrating contactpotential difference sensor 101, a system 103 for mechanically fixturingthe wafer 105, a system 107 for positioning the sensor 101 a fixeddistance above the wafer surface 106 and generating relative motionbetween the probe tip 102 and wafer surface 106 such that the sensorprobe tip 102 moves parallel to the wafer surface 106, a source ofillumination 109 with variable intensity, angle, or spectrum of lightthat can irradiate the semiconductor wafer surface 106 under or near thesensor probe tip 102, a source of charge 110 that can deposit a knownamount of charge on the wafer surface 106 prior to scanning by thesensor probe tip 102, a system 104 for vibrating the probe tipperpendicular to the wafer surface, and a system 112 for acquiring andprocessing the output signal from the sensor 101 to identify andclassify wafer 105 non-uniformities.

In one embodiment, a semiconductor wafer 105 is placed on a conductivewafer fixture 103. This may be done manually or using an automatedprocess such as, but not limited to, a wafer handling robot. The wafer105 is held in place such as by using vacuum. Alternative methods ofholding the wafer 105 include, but are not limited to, electrostaticforces and edge gripping. In one embodiment, the fixture 103 is mountedto a spindle which can rotate the wafer 105 about its center. Thenon-vibrating contact potential difference sensor 101 is attached to apositioning system 107 that can adjust the height of the sensor 101above the wafer surface 106 and can move the sensor 101 radially from atleast the center of the wafer 105 to one edge of the wafer 105. Thenon-vibrating contact potential difference sensor 101 is electricallyconnected to the wafer surface 106 via the conductive wafer fixture 103.This connection can be resistive or capacitive. In one embodiment, aheight sensor 111 that has been calibrated to the height of thenon-vibrating contact potential difference sensor probe tip 102 is alsomounted on the same positioning system 107 as the non-vibrating contactpotential difference sensor 101.

Alternately, a light source 109 with variable intensity, wavelength orangle may be mounted on the positioning system such that the illuminatedarea includes at least the area next to the non-vibrating contactpotential difference sensor probe tip 102. Alternately, a light sourcecan be mounted separate from the positioning system so that itilluminates at least the entire area scanned by the non-vibratingcontact potential difference sensor. Alternately, a source of controlledcharge may be mounted on the positioning system so that a known amountof charge can be deposited on the wafer surface, or on or in adielectric layer present on the wafer, prior to, or even during,scanning by the non-vibrating contact potential difference sensor.

In an alternate embodiment, a system 104 for vibrating the sensor 101perpendicular to the wafer surface 106 is attached to the contactpotential difference sensor. This system is used to make vibratingKelvin probe measurements of the contact potential difference betweenthe probe tip 102 and the wafer surface 106.

After the wafer 105 is secured to the fixture, the height sensor 111 ispositioned above one or more points on the wafer surface 106 and theheight of the wafer surface 106 is measured. These wafer heightmeasurements are used to calculate the position of the non-vibratingcontact potential difference sensor 101 that will produce the desireddistance between the probe tip 102 and the wafer surface 106. Thisinformation is used to position the probe tip 102 at a fixed heightabove the wafer surface 106, and the probe tip 102 is moved to a pointabove the outside edge of the wafer 105. Illumination of the wafersurface may be enabled and the appropriate intensity and wavelengthselected for the inspection application. Also, the appropriate amount ofcharge may be selected for the inspection application.

As shown for example in FIG. 2A, the probe 101 is held stationary andthe wafer 105 is rotated on the spindle such that the probe tip 102moves relative to the wafer 105 along a circular path that is centeredat the wafer 105 center. Data is acquired during a single rotation ofthe wafer 105. The sensor 101 is then moved a programmable distancealong the radius of the wafer 105 towards the wafer center. Anotherrotation of data is acquired at this new radius. The probe tip 102continues to step and scan concentric circular regions of the wafer 105until the probe reaches the wafer 105 center. The resulting data is thenassembled into an image of the wafer 105. Alternately, each concentriccircular region of the wafer 105 could be scanned multiple times and theresulting data averaged to reduce the effect of random noise. This imageis processed to identify and classify non-uniformities (see FIG. 2B).This processing can take many forms. It may be as simple as thethresholding of signal values to detect regions of the wafer surface 106where the surface potential is changing (compare FIGS. 3A and 3B). Thedifferential sensor data can also be integrated to generate an imagewhich represents relative surface potential values. This integratedimage can also be processed or thresholded to identify regions withspecific relative potential values. Alternately the integrated image canbe calibrated using multiple vibrating Kelvin probe measurements toproduce an image with values that represent the absolute contactpotential difference value at each point on the wafer. This image canthen be thresholded to identify regions with specific contact potentialdifference values.

FIG. 2A show a diagram of the radial scanning of one embodiment of thepresent invention. The non-vibrating contact potential difference sensorprobe tip 102 is positioned at point “A” near the edge of the wafer 105.The wafer 105 is rotated on the wafer fixture 103 and a circular trackof data is scanned. The probe tip 102 is moved a programmable distancetowards the wafer 105 center to point “B” and a second circular track ofdata is scanned. This process is repeated until the probe tip 102reaches the center of the wafer 105. The resulting data is combined intoan image of the wafer surface 106. A sample image is shown in FIG. 2B,and the light and dark regions representing various non-uniformities.

One aspect of this invention is the identification or classification ofnon-uniformities based on the polarity of the surface potential orcapacitance change. The differential non-vibrating contact potentialdifference sensor signal can be integrated to form a signal thatrepresents relative surface potential or capacitance values. As shown inFIGS. 3A and 3B, this integrated signal can be thresholded to identifyregions with signal values that lie within a specific range, or areabove or below specified values. As shown in FIGS. 4A-4C, anon-uniformity can be classified as having a positive or negative signbased on whether the integrated signal is higher or lower than theintegrated signal values for the surrounding wafer surface, or it can beclassified as positive or negative based on whether the integratedsignal value is higher or lower than the average integrated signal valuefor some portion of the wafer surface. The integrated image showsregions of relative surface potential. Some regions are brightindicating areas of FIG. 4A with increased surface potential or workfunction and some regions are dark indicating areas of decreased surfacepotential or work function. In this case the bright regions are metalpatterns on the wafer and the dark regions are non-metal or organiccontaminants on the wafer. FIG. 4B is the image from FIG. 4A afterapplying a threshold to identify areas of increased surface potential.The dark regions show areas of the image which are greater than apositive threshold. These regions represent areas of increased surfacepotential or work function and correspond to the metal pattern. Theimage of FIG. 4C is the image of FIG. 4A after thresholding to identifyregions of decreased surface potential. Dark regions show areas of theimage which are less than a negative threshold. These regions representareas of decreased surface potential or work function, and correspond toregions of non-metallic contamination. For non-uniformities resultingfrom changes in the height of the wafer surface, the polarity of thenon-uniformity indicates whether the wafer height at the non-uniformityis higher or lower than the surrounding area. For non-uniformitiesresulting from changes in surface potential, the polarity of thenon-uniformity indicates if the surface potential is higher or lower atthe non-uniformity. This information can be used to classify differenttypes of chemical non-uniformities. For example, contamination of asilicon surface with some types of metals will result in an increase insurface potential, while contamination with many organics will result ina decrease in surface potential. In this case it is possible todiscriminate between metal and organic contaminants based on thedirection of surface potential change. Independent data from othersensors of conventional nature can be readily used to infer likely typesof non-uniformity. In addition, the basic nature of a manufacturingprocess will provide clues as to the category of non-uniformity.

A second aspect of the invention relates to the discrimination andclassification of non-uniformities as resulting from changes incapacitance between the probe tip and the wafer surface or resultingfrom changes in potential between the probe tip and the wafer surface(see, for example, FIGS. 9A-9D). Features in the image include bothchemical non-uniformities and height changes. The highlighted feature inFIG. 9B is surface contamination which causes a change in surfacepotential on the wafer. The feature highlighted in FIG. 9C is a changein wafer height resulting from a particle trapped between the wafer andthe vacuum chuck used to secure the wafer for scanning. The trappedparticle causes a local increase in the height of the wafer surface,which results in a change in capacitance between the wafer surface andthe probe tip. The signal resulting from the change in surface potentialof FIG. 9B is relatively unchanged from the image of FIG. 9A, but thesignal from the change in capacitance is amplified by the bias voltage.This is illustrated by the details of the image of FIG. 9B, which isformed by subtracting the image of FIG. 9A from the image of FIG. 9B.The surface potential feature is removed from the image by thesubtraction operation but the change in capacitance feature remains.This technique is used to detect and classify height non-uniformities onthe wafer, or to discriminate height non-uniformities from surfacepotential non-uniformities. The resulting scan in FIG. 9D clearly showschemical non-uniformities, but height non-uniformities are largelyeliminated. This technique is used to detect and classifynon-uniformities resulting from changes is surface potential.

The wafer may be scanned twice with different bias voltages and oneimage subtracted from the other to form a new image that minimizes thesignal resulting from surface potential non-uniformities and increasesthe signal resulting from wafer height non-uniformities.Non-uniformities which are detected in the resulting image areclassified as height non-uniformities (see also FIGS. 6A-6C). FIGS.6A-6C show three images, and the image of FIG. 6A was acquired byscanning a wafer with a non-vibrating contact potential differencesensor with a 5 Volt bias applied to the probe tip. The non-uniformitiesin the upper right and lower left quadrants of the wafer are etchedtrenches in the silicon. The signal from these features is generated bychanges in the height of the wafer surface which induces a capacitancechange between the sensor probe tip and the wafer. The non-uniformitiesin the lower right quadrant are a thin film of metal. The signal fromthese features is generated by a change in surface potential; the imageof FIG. 6B is an image of the same wafer with a −5 Volt bias applied tothe probe tip,and the image of FIG. 6C is the difference of the imagesof FIGS. 6A and 6B. The etched trenches are clearly visible; but theother features, which result from surface chemistry changes, areeliminated. These images demonstrate the use of subtracting two imagesacquired with different bias voltages to identify features that arecaused by height non-uniformities. In addition, a vibrating Kelvin probemeasurement of the contact potential difference between the probe tipand the wafer surface can be made at one or more points on a wafer. Thismeasurement can determine the average contact potential differencebetween the probe tip and the wafer surface; or the average contactpotential difference of a particular type of wafer can be determined bymeasuring one or more wafers of that type. A bias voltage that is equalin magnitude but opposite in sign to this average contact potentialdifference can be applied to subsequent scans to minimize the signalresulting from changes in capacitance between the probe tip and thewafer surface. Non-uniformities in the resulting image are classified aschanges in wafer surface potential as opposed to changes in capacitance.

A third aspect of this invention is the identification or classificationof non-uniformities based on shape or signal value statistics. Afternon-uniformities in or on the wafer have been identified from thenon-vibrating contact potential difference image, or from an integratedversion of the image, then these non-uniformities can be classifiedbased on features extracted from the differential image, the integratedimage or from an integrated image scaled to vibrating Kelvin probemeasurements. These features can consist of values describing the shapeof the non-uniformity; such as area, perimeter, height and width; or thesignal values associated with the non-uniformity; such as standarddeviation, maximum, minimum, range, and average. In addition, featuresdescribing the position of the non-uniformity on the wafer or therelative positions of non-uniformities can be used to classifynon-uniformities.

A fourth aspect of this invention is the classification ofnon-uniformities based on the sensitivity of the non-uniformity to lightor surface deposited charge. Some types of non-uniformities, such asdoping or implant non-uniformities, or charging in or on a dielectricfilm on or in a semiconductor substrate, are sensitive to surfaceillumination or charging. Two images can be acquired with differentsurface illumination conditions and the images subtracted. The resultingimage shows changes in surface potential with illumination (see FIGS.5A-5C). FIGS. 5A-5C show three images, and the image of FIG. 5A wascreated by scanning a wafer with a non-vibrating contact potentialdifference sensor with no surface illumination; FIG. 5B was created byscanning the same wafer with the surface illuminated by visible light;and FIG. 5C is the difference between the images of FIG. 5A and FIG. 5B.The features that are visible in the image of FIG. 5C were created byimplanting dopants in the wafer. Images acquired with different levelsof super bandgap illumination can be subtracted and used to detect orclassify defects which are sensitive to illumination, such as doping orimplant non-uniformity, or charging in or on a dielectric film on asemiconductor substrate. This image is sometimes called a Surface PhotoVoltage (SPV) image. The surface photo voltage image can be processed toidentify and classify non-uniformities that are sensitive to light, suchas implant non-uniformities or dielectric charging. Likewise, two imagescan be acquired with different charge conditions and subtracted. Theresulting image can be used to detect non-uniformities that aresensitive to surface charge conditions.

A fifth aspect of this invention is the classification ofnon-uniformities based on the results of subsequent vibrating contactpotential difference measurements. The non-vibrating contact potentialdifference sensor produces differential data that represents changes insurface potential or height across the wafer. Vibrating Kelvin probemeasurements, however, provide a measure of the absolute contactpotential difference between the sensor probe tip and the wafer surface(see FIG. 8). Measurements were made on a silicon surface contaminatedwith both metallic and organic contaminants. The organic contaminantsresult in lower contact potential difference values than the metalliccontaminants. Once non-uniformities have been identified and locatedusing the non-vibrating sensor image, or an integrated version of theimage, then vibrating Kelvin probe measurements can be made at thelocations of the non-uniformities (see, for example, FIG. 7). Featuresin each quadrant of FIG. 7 were created by applying different metals tothe surface of the wafer. Vibrating contact potential difference sensormeasurements were then made at the locations of non-uniformitiesidentified in each quadrant. The vibrating contact potential differencemeasurements show different surface potential values for each of themetals, illustrating how vibrating contact potential difference sensormeasurements can be used to classify defects detected by thenon-vibrating contact potential difference scanned image. The contactpotential difference as measured by the vibrating Kelvin probe usuallyvaries with surface chemistry and these measurements can provideinformation that is useful in classifying surface non-uniformities. Inaddition, some types of non-uniformities, such as charges (deposited orotherwise present) trapped in or on a dielectric deposited on thesurface of the wafer, can create relatively large contact potentialdifference measurements. Charging non-uniformities that producerelatively large signals can be classified based on the magnitude of themeasurement and also on changes of the level of signal. For example, acontact potential difference measurement of over one Volt is often dueto charging in a dielectric.

A sixth aspect of this invention is the classification ofnon-uniformities based on contact potential difference values obtainedby scaling integrated non-vibrating contact potential difference imageto two or more vibrating contact potential difference measurements (seeFIG. 7). The resulting image values represent approximate contactpotential difference values at each point in the image. These values canbe used much like vibrating contact potential difference measurements toclassify non-uniformities based on absolute contact potential differencevalues. The resulting image is useful for identifying regions withspecific surface potential values, and is particularly useful fordetecting and classifying areas of large contact potential differencethat typically result from charging in or on a dielectric.

There are many alternate mechanical configurations and scanningoperations that would accomplish the same result as the embodimentdescribed above. For example, the non-vibrating contact potentialdifference sensor 101, height sensor 11 1, illumination source 109,charge source 110, and system for vibrating the sensor 104 could all bemounted at fixed locations, and the wafer 105 could be moved and rotatedbeneath these stationary elements. Instead of stepping from one radiusto the next, the non-vibrating contact potential difference sensor 101could be moved continuously along the wafer 105 radius while the wafer105 is spinning to create a continuous stream of data that spiralsacross the whole surface of the wafer 105. Also, instead of the radialscanning operation described above, the non-vibrating contact potentialdifference sensor 101 could be moved linearly across the wafer 105 in aback-and-forth manner to scan the entire wafer surface 106. Also,multiple non-vibrating contact potential difference sensors andillumination sources could be used to acquire multiple measurementssimultaneously to reduce the time required to measure a wafer.

The foregoing description of embodiments of the present invention havebeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the present invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of thepresent invention. The embodiments were chosen and described in order toexplain the principles of the present invention and its practicalapplication to enable one skilled in the art to utilize the presentinvention in various embodiments, and with various modifications, as aresuited to the particular use contemplated

1. A method of classifying a non-uniformity of a semiconductor wafer,comprising the steps of: providing a semiconductor comprising a waferhaving a wafer surface; providing a contact potential difference sensorhaving a probe tip; scanning the wafer surface laterally relative to thecontact potential difference sensor; generating sensor datarepresentative of changes in contact potential difference between thesensor probe tip and the wafer surface as the sensor probe tip scanslaterally relative to the wafer surface; using the contact potentialdifference sensor data to detect wafer non-uniformities; processing thecontact potential difference sensor data at the location of anon-uniformity to detect a trend to one of lower or higher surfacepotential of the wafer surface; and using the trend to one of lower orhigher of surface potential as one feature to classify the wafernon-uniformity.
 2. The method as defined in claim 1 further includingseparating the sensor data between spatial change and chemical change onthe wafer surface;
 3. The method as defined in claim 1 where thenon-uniformity is classified as either metal or non-metal;
 4. The methodas defined in claim 1 further including the step of providingindependent data characteristic of the likelihood of the chemical changebeing one of metal contamination and non-metal contamination and usingthe independent data and the trend of surface potential to analyze thetype of non-uniformity;
 5. The method as defined in claim 1 wherein anincrease in surface potential is indicative of a metal contamination anda decrease of surface potential is indicative of an organiccontamination.
 6. The method of claim 1, further including applying abias voltage to the sensor probe tip or the wafer surface to minimizeaverage differences in the potential difference sensed, therebyminimizing the signal component arising from changes in distance betweenthe probe tip and the wafer surface.
 7. The method of claim 2 furtherincluding the steps of varying a bias voltage applied between the probetip and the wafer surface, wherein the change in the sensor dataresulting from varying the bias voltage applied to the probe tip orwafer surface is used to classify the non-uniformity as resulting from achange in one of wafer height and a change in chemical state of thewafer surface.
 8. The method of claim 6 wherein the bias voltage thatminimizes the average contact potential difference between the probe tipand the wafer surface is calculated as the average of at least onevibrating Kelvin probe measurement made on a representative test wafersurface.
 9. The method of claim 1 further including the step of applyingan illumination to the wafer surface.
 10. The method of claim 1 furtherincluding the step of characterizing shape of a curve of the sensor datato correlate to a particular type of the non-uniformities.
 11. Themethod of claim 1 further including the step of classifying anon-uniformity by vibrating a Kelvin probe at the location of thenon-uniformity and sensing an associated signal.
 12. The method of claim11 further including the step of classifying a non-uniformity bymeasuring charge present at least one of on the wafer surface, on adielectric film present on the wafer surface, and in the dielectric filmpresent on the wafer surface based on the magnitude of a measurement ofvibrating the Kelvin probe.
 13. The method of claim 11 wherein the atleast one vibrating Kelvin probe measurement is compared to vibratingKelvin probe measurements on known contaminants to classify thenon-uniformity.
 14. The method of claim 11 wherein the step ofclassifying a non-uniformity comprises measuring the sensor data at alocation of a non-uniformity in an image that is formed by integratingthe non-vibrating contact potential difference sensor data andtransforming resulting integrated data into absolute contact potentialdifference values based on at least two vibrating Kelvin probemeasurements on the wafer surface.
 15. A system for classifyingnon-uniformity type of a semiconductor wafer comprising: a potentialdifference sensor to produce a sensor signal; a wafer fixture to hold asemiconductor wafer and a mechanism to scan the wafer relative to thepotential difference sensor and to rotate a spindle coupled to the waferfixture, thereby enabling relative motion between the wafer and thesensor; and a source of independent data indicative of whether anon-uniformity type is a metallic contaminant or nonmetallic contaminantbased on a trend of the sensor signal to a positive or negative changeof potential difference, thereby enabling determination of thenon-uniformity type on the wafer.
 16. The system as defined in claim 15further including a bias voltage source for applying a bias voltage toat least one of the wafer and the sensor, thereby enabling minimizingaverage differences in potential differences sensed to minimize a signalcomponent arising from changes in distance between the sensor and asurface of the wafer.
 17. The system as defined in claim 15 furtherincluding a vibrating Kelvin probe which measures a vibrating probepotential difference at selected locations of a non-uniformity.
 18. Thesystem as defined in claim 17 further including a device to depositcharge on a surface of the wafer, thereby enabling the classification ofa non-uniformity by measuring a change in signal induced by the chargedeposited on the surface of the wafer by use of the vibrating Kelvinprobe.
 19. The system as defined in claim 15 further including a sourceof light which can be applied to the wafer at selected angles ofinclination with variable light intensity and variable wavelength,thereby enabling further probing of the wafer.
 20. The system as definedin claim 19 wherein the source of light can be controlled to vary atleast one of the light intensity and the wavelength for classifying thenon-uniformity of the wafer of the semiconductor wafer.
 21. A system forclassifying non-uniformity type of a semiconductor wafer comprising: apotential difference sensor to produce a sensor signal; a wafer fixtureto hold a semiconductor wafer and a mechanism to scan the wafer relativeto the potential difference sensor and to rotate a spindle coupled tothe wafer fixture, thereby enabling relative motion between the waferand the sensor; and a source of data indicative of whether anon-uniformity type is one of a metallic contaminant, nonmetalliccontaminant and excess electronic charge based on the type of sensorsignal, thereby enabling determination of the non-uniformity type on thewafer.